Fluorite structure superstructures

In 1923 CeC>2 was found to have the fluorite structure (Goldschmidt and Thomas-sen, 1923), and in 1926 PrC>2 was found to have the same structure (Goldschmidt, 1926). In 1950 the intermediate compositions in the PrO system were reported to have the fluorite structure of variable lattice parameters (the superstructure reflections were not observed). In 1951 TbC>2 was also shown to have the fluorite structure (Gruen et al., 1951). By this time the basic fluorite structure had been established for all the known higher oxides of cerium, praseodymium and terbium. 

A considerable number of fluorides oxides, and oxyfluorides have structures which are related more or less closely to the fluorite structure. The relation is closest for compounds with cation anion ratio equal to 2 1 with random arrangement of cations or anions, these structures having the normal cubic unit cell examples include the high-temperature forms of NaYp4 and K2UF6 and the oxyfluorides AcOF and HoOF. As in the case of the NaCl structure there are tetragonal and rhombohedral superstructures, which are described in Chapter 10. These are the structures of a number of oxyfluorides ... 

In addition to the binary nitrides described here, some ternary nitrides have been prepared, for example, Li5TiN3, Li7NbN4, and Li9CrNs, all with the fluorite type of structure (superstructures in most cases), and alkaline-earth compounds with Re, Os, Mo, or W(e.g. Sr9Re3Nio, CasMoNs). ... 

As noted earlier Th(iii) is known only in the solid state, in ThCla, ThBr3, and Th2S3 (Table 28.2), and oxyfluorides. The latter include ThOF (statistical fluorite structure) and ThOo.sF2.s (superstructure of LaF3 type),... 

There are three different kinds of arrays for the superstructures. One superstructure is oriented along the (135)i direction another is in the (220) direction. The last occuring in the phase with n=62 and m=6 has 62 spots in the (12,6,26) and (22,20,4)f of the fluorite structure. These diffraction patterns also indicate that the short axes (a w 6.5 A) of the superstructures unit cell is along the beam orientation (112) , except again for the phase with n=62 and m=6. 

Fig. 10. Constituent structural elements of rhombohedral fluorite-related superstructures with anion excess (AFg cube, icosahedron, and A R F37,.24 clusters, seen from the outside and from the interior).

The actual atomic arrangements are known for KH02F7 (Le Fur et al., 1982), KEr2F7 (A16onard et al., 1980), and KYb2F7 (Le Fur et al., 1980). The first structure type is a fluorite-related superstructure with the cation sequence -ABC-. Ordering of cations takes place in A and B with the ratio lK 3Ho, but lK lHo in C. The coordination number of Ho " is eight in all cases one quarter... 

It may be noteworthy that in both cases, Ba2RCl7 and BaRCls, a structural change from the rather complicated three-dimensional fluorite-type superstructures to well defined compounds occur. 

In some layered compounds, electric charge separation between the layers may destabilize the structures AurivilUus phases [Bi202] [B lM 03, l] " are examples. If oxygen vacancies are introduced into an Aurivillius phase, a stepped superstructure may form to release the charge separation and, probably, reduce the number of oxygen vacancies. For example, Bi2WO,5 is a well known n = I member of the family of Aurivillius phases. The unit cell consists of fluorite-like [Bi202] and... 

The fluorite superstructure of 7-Na2UF6 is illustrated in Fig. 28.3 (p. 995). There is slightly distorted cubic coordination of M in Na3UF8 (and the isostructural Na3PuF8). Owing to the arrangement of cations in this structure (Fig. 6.10) the unit cell is doubled in one direction and is a b.c. tetragonal cell. 

---